Ribonucleotide reductase catalyzes for the formation of deoxyribonucleotides, the precursors of DNA, by reduction of the 2' -OH group of ribonucleotides. The enzyme is allosterically regulated by the nucleoside triphosphates of the cell. In addition, the activity of the enzyme is related to growth rate and cell cycle. Because of the importance of this enzyme in the control of deoxyribonucleotide formation, DNA synthesis, and cell proliferation, an understanding of its mechanisms of action and of regulation is needed. Achievement of this goal will require the purification of the enzyme and the localization and structural analysis of the active sites and allosteric effector binding sites. In addition physical measurements will be needed to determine the size of the enzyme and its subunits, the effect on physical properties of allosteric effector binding, and the properties of the enzyme reaction. Ribonucleotide reductase from mammalian source will be purified by use of affinity chromatography and physico-chemical separation methods. The active and regulatory site regions will be identified by affinity labeling and the structure of these regions determined by amino acid sequence analysis. The enzyme and its subunits will be characterized physically, spectrally, and by enzyme kinetics approaches.